The present invention pertains to medical catheters which can be used to deliver a stent to a deployment site within the vascular system of a patient. More specifically, for stent delivery within the cardiovascular system of a patient.
Stents are devices deployed in the cardiovascular system of a patient to maintain the patency of a vessel at the site of a lesion, or stenosis. Typically, this requires advancement of the stent through the cardiovascular system and then deployment of the stent at a stenosis site in the vessel. A balloon expandable stent is delivered by securing it onto a balloon of a delivery catheter which then may be advanced through the vascular system to the stenosis site. Once at the stenosis site, the balloon is inflated to deploy the stent.
Significantly, the delivery catheter must bend in different directions as it follows one of the tortuous routes through the vascular system to the stenosis site. As the catheter bends, the attached stent will also bend, and the ends of the stent may deform and flare outwardly from the balloon, thereby increasing the profile of the stent. With such an increased profile the stent may not advance further through the cardiovascular system to cross the lesion site, or the stent may not be easily withdrawn from the body, if that is desired. It is preferred that the stent be retained snugly against the balloon until the stenosis site is reached. Once the stent is placed across the stenosis, the stent should quickly and easily separate from the catheter after the balloon inflates during the deployment of the stent.
Various devices have been proposed to retain the stent against the balloon of a delivery catheter. Several prior art retention devices incorporate pairs of elastic cuffs, or caps, which are placed over the stent proximal and distal ends to retain the ends against the catheter. For these devices, the ends of the stent must slide out from under the end caps as the balloon expands the diameter of the stent. However, the stent may fail to completely exit from within a cap during deployment, possibly leaving the cap caught between the stent and the vessel wall after the balloon has been deflated. To avoid this problem, the end caps may be mounted with only a short overlap of the stent ends, which can lead to premature uncovering of the stent ends and concomitant loss of retention.
In another prior art device, retention sleeves self-retract from their positions overlying the ends of the stent during inflation of a balloon. The sleeves are anchored to a catheter shaft, and they need to fold or accordion to reduce their overall length while sliding down the cones of the expanding balloon. In another prior art example, the ends of a balloon-mounted stent are overlaid by cuffs formed from excess material of the balloon.
It is an object of the present invention to provide a balloon catheter for delivering a stent which retains the stent snugly against the catheter balloon during its advancement into the vascular system.
The delivery catheter of the present invention includes an elongate shaft and an inflatable tubular balloon that is bonded to the distal end of the catheter shaft. With the balloon deflated, a cylindrical stent is crimped onto the balloon, and proximal and distal end caps are provided to retain the stent on the balloon until it is deployed. The balloon is mounted on the catheter shaft such that excess balloon material can be gathered, or folded, beyond the ends of the stent beneath the end caps. During inflation of the balloon, the excess balloon material unfolds, allowing the balloon to elongate. The portion of the shaft that extends within the balloon is axially stretchable, either elastically or plastically, to accommodate the lengthening of the balloon. The proximal and distal end caps are anchored to the catheter shaft such that the lengthening of the balloon and the shaft portion during inflation will cause the caps to axially separate with the ends of the balloon, thus uncovering the ends of the stent. The invention features a reliable mechanism to withdraw the retention caps from the ends of the stent during deployment, such that the caps of the present invention can cover wider margins at the ends of the stent than were previously advisable, thus providing more dependable retention of the stent on the catheter.
The balloon of the invention is stretch blow-molded from a high-strength thermoplastic material, as is well known in the art of balloons used for dilatation and/or stent delivery. The proximal and distal caps are made of a soft elastic material, preferably a thermoplastic elastomer, which can be heat treated to enhance the ability of the caps to retain the stent against the balloon. The heat treating process for the end caps can be performed using conventional heat-shrink tubing to set the shape of the caps and to partially embed portions of the caps into the distal and proximal margins at the ends of the stent. End caps thus molded establish a firm grip on the stent margins. Heat treating the end caps can also reduce the profile of the assembly to facilitate advancement of the delivery catheter through the cardiovascular system of a patient.
In use of the present invention, the catheter is advanced through the cardiovascular system of a patient until the stent at the distal end of the catheter is positioned across the target lesion. Next, the balloon is inflated to simultaneously expand the stent and to retract the end caps from their positions covering the ends of the stent. With the stent expanded and compressed against the vessel wall, the balloon is deflated to contract it and separate it from the stent. The elastic proximal and distal caps contract with the deflating balloon and will surround the ends of the balloon, when collapsed. If the shaft portion within the balloon has undergone elastic elongation, the balloon will substantially return to its former longitudinally compressed configuration with excess portions. Alternatively, if the distal shaft has undergone plastic deformation during elongation, the balloon will merely collapse onto the catheter shaft. In either event, the delivery catheter, with its reduced deflated balloon profile, can be withdrawn from the patient while the stent remains deployed in the patient""s vessel.